Consider using exceptions instead of null values to signal invalid plugins during discovery

In some cases, the invalid plugin is expected -- such as the case handled by 🐛 Changing plugins from annotations to attributes in contrib leads to error if plugin extends from a missing dependency Needs review . I thought using exceptions as messages was a bad pattern?

Well the code just does this:

              ['id' => $id, 'content' => $content] = $this->parseClass($class, $fileinfo);

              if ($id) {
                $definitions[$id] = $content;
                // Explicitly serialize this to create a new object instance.
                $this->fileCache->set($fileinfo->getPathName(), ['id' => $id, 'content' => serialize($content)]);
              }
              else {
                // Store a NULL object, so the file is not parsed again.
                $this->fileCache->set($fileinfo->getPathName(), [NULL]);
              }

So really it's just replacing that $id check with a catch? Or, we also allow parseClass to return just NULL directly instead of an array of NULLs?

My 0.02$: throwing/catching something like an InvalidPluginClassException would be best - clean, unambiguous, testable